Shell solid, equivalve; inequilateral, beaks in the anterior half; somewhat broadly oval in outline. Ligament inset, not concealed, a thick brown elliptical arched body extending almost half-way back to the posterior margin. Lunule elongate heart-shaped, clear though not particulary well defined, with light and dark brown fine radiating ridges. Escutcheon reduced to a mere border of the posterior region of the ligament. Sculpture of radiating ribs and concentric grooves, the latter becoming particulary sharp over the anterior and posterior parts of the shell, making the surface pronunced decussate. Growth stages clear.
Three cardinal teeth in each valve; centre tooth in left valve and centre and posterior in right, bifid. No lateral teeth.
Pallial sinus relatively deep though not extending beyond the centre of the shell; it leaves a wedge-shaped space between its lower limb and the pallial line. Margin smooth.
Extremely variable in colour and pattern, white, yellow or light brown, sometimes with rays, steaks, blotches or zig-zags of a darker brown, slightly polished; inside of shell polished white with an orange tint, sometimes with purple over a wide area below the umbones.

Originate from the southeastern Asia (Indo-Pacific). Introduced for commercial purposes in the Mediterranean (only Adriatic Sea and tentatives of aquaculture in Tyrrenian Sea) and Brittany, France, where it lives in the same habitat as Ruditapes decussatus. Has also been introduced in the Hawaiian Islands and along the Pacific coast of northern America.

Habitat and Biology

Lives in sand, muddy gravel or stiff clay, below midtide level and littoral lagoons.Feeds on plankton and micro-organisms that are carried along the bottom by currents.

Size

Maximum length is 8 cm; common 2,5-5,7 cm.

Interest to Fisheries

Artesanal or semi-industrial fishery. Caught with dredges and hackles, occasionally with bottom trawls. The total catch reported for this species to FAO for 1999 was 57 051 t. The countries with the largest catches were Japan (43 088 t) and Korea, Republic of (13 963 t).

Local Names

English :
Short necked clam .

French :
Palourde japonaise .

German :
Japanische muschel .

Spanish :
Almeja japonesa .

Remarks

First introduced in 1985 in Italy by the "sowing" of more than 10,000,000 specimens in their larval stage. Four years later, adults could be collected. Increasingly common in European waters. Similar to Ruditapes decussatus but differs from it by the much more pronunced dessate sculpture and colour.